1. Field of the Invention
The present invention relates in general to semiconductor device manufacturing and, in particular, to apparatus and procedures for checking the vacuum integrity of ion implanters.
2. Description of the Related Art
The manufacturing of semiconductor devices often involves the processing of a semiconductor substrate (e.g., a silicon wafer) through a series of processes including the implantation of dopant ions into the semiconductor substrate. Such ion implantation processes are typically performed in an ion implanter that has been pumped down to a high vacuum state.
FIG. 1 is a sketch illustrating the subsystems of a conventional ion implanter 10. The subsystems include an ion beam line apparatus 12 for producing, analyzing and accelerating dopant ions for implantation, a disk 14 for holding semiconductor substrates and an end station chamber 16 for loading, holding and positioning disk 14. Ion implanter 10 also includes a high vacuum system 18 for evacuating ion beam line apparatus 12 and end station chamber 16, a gas supply system 20 and a computer control system (not shown) for managing the operation of ion implanter 10. See S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era, Volume 1: Process Technology, 308-317 (Lattice Press 1986) for a further description of ion implanters.
Gaseous contaminants can be introduced into an ion implanter through disruptions (e.g., a small leak) in the ion implanter""s vacuum integrity. The gaseous contaminants can then interfere with the ion implantation process by, for example, colliding with the dopant ions. The result is undesirable variation in the electrical characteristics of the implanted semiconductor substrates. Since the electrical characteristics are typically measured after the semiconductor devices have been fully manufactured, resources can be wasted in processing semiconductor substrates that do not meet electrical requirements.
Conventional ion implanters employ two stored pressure set points: one pressure set point for controlling the ion implanter at the ion implantation process pressure (e.g., 8.5 E-5 Torr) and the other for defining an over-pressure condition (e.g., 1.4 E-4 Torr). Disruptions in vacuum integrity caused by small leaks can, however, only be detected at pressures significantly below the ion implantation process pressure and over-pressure set points. Typically, such disruptions in vacuum integrity are detected at a pressure in the vicinity of, e.g., 1.0 E-5 Torr.
Conventional ion implantation processes involve pumping down (i.e., evacuating) an ion implanter to the ion implantation process pressure and, then, implanting semiconductor substrates at that pressure. The ion implantation process pressure is maintained by introducing a high purity gas into the end station chamber of the ion implanter. The pressure in the end station chamber is monitored during the ion implantation process and compared to the over-pressure set point. If the pressure rises to the over-pressure set point (e.g., due to excessive out-gassing from photoresist on the semiconductor substrates or a large leak in the ion implanter), the ion implantation process is stopped until the ion implanter can again be pumped down to the ion implantation process pressure. A drawback of this conventional ion implantation process is its inability to attain the low pressures needed to adequately check vacuum integrity for the presence of small leaks.
A vacuum integrity check process that enables adequate detection for the presence of small leaks would entail pumping down the ion implanter to a vacuum integrity check pressure (e.g., 1.0 E-5 Torr) that is significantly lower than the ion implantation process pressure. This can be accomplished by changing the ion implantation process pressure set point to the lower vacuum integrity check pressure set point. If the vacuum integrity check pressure can be reached within a predetermined time period, then vacuum integrity is confirmed and the ion implanter is considered leak free. However, since conventional ion implanters can not store both a vacuum integrity check pressure set point and an implantation process pressure set point, an ion implantation process can not be performed following the vacuum integrity check. Vacuum integrity checks at pressures significantly below the ion implantation process pressure are, therefore, only performed on an infrequent basis (e.g., once every ten ion implantation processes). This exposes those semiconductor substrates implanted between the vacuum integrity checks to the risk of implantation in the presence of gaseous contaminants from small leaks. In addition, simply pumping down the ion implanter to the vacuum integrity check pressure takes an undesirably long time (e.g., 15 minutes) and is ineffective in removing contaminants from within the ion implanter.
Still needed in the field, therefore, are an ion implanter vacuum integrity check process and apparatus that enable a vacuum integrity check at a pressure significantly below the ion implantation process pressure, while storing an ion implantation process pressure set point for a subsequent ion implantation process. In addition, the vacuum integrity check process and apparatus should provide for the effective removal of contaminants from within the ion implanter and be of a short duration.
The present invention provides an ion implanter vacuum integrity check process and apparatus that enable a vacuum integrity check at a pressure significantly below the ion implantation process pressure, while storing an ion implantation process pressure set point for a subsequent ion implantation process. In addition, the vacuum integrity check process and apparatus provide for the effective removal of contaminants from within the ion implanter and is of short duration.
Processes in accordance with the present invention include first providing an ion implanter. The ion implanter includes an end station chamber, a high vacuum system, a disk, a gas supply system and a controller for storing at least one vacuum integrity check pressure set point and one implantation process pressure set point. The disk is, then, inserted into the end station chamber, which is subsequently sealed. Next, the disk is accelerated to a predetermined rotational speed (e.g., 953 rpm) using the controller, while the high vacuum system is used to pump down the end station chamber. Once the disk reaches the predetermined rotational speed, the end station chamber is purged with an inert gas (e.g., high purity nitrogen or argon) at a first predetermined flow rate (e.g., 15 sccm) for a first predetermined time period (e.g., 15 seconds). During this purge step, the predetermined rotational speed of the disk is maintained and the high vacuum system continues to pump down the end station chamber. Next, the pressure of the end station chamber is monitored, while maintaining the predetermined rotational speed and continuing to pump down the end station chamber. A determination is, then, made as to whether or not the pressure within the end station chamber had reached the vacuum integrity check pressure set point within a second predetermined time period. If the vacuum integrity check pressure set point had been reached, then the ion implanter is considered to have acceptable vacuum integrity. Failure for the vacuum integrity check set point to be reached within the second predetermined time period is evidence of a disruption of the ion implanter""s vacuum integrity, indicating the presence of a leak.
An apparatus according to the present invention includes a controller for storing at least one each of a vacuum integrity check pressure set point, an ion implantation process pressure set point and an over-pressure set point.